An expectation of realization of a cognitive radio network has been increasing. A cognitive radio network enables configuration of a communication network fit for an ambient environment, because each of node devices configuring the communication network performs communication by selecting one or more channels from among a plurality of candidates of a utilizable radio communication frequency (hereinafter, referred to as a channel) based on the ambient environment (an unused channel, a distance between node devices, a required communication bandwidth (a communication demand), and so on).
In a cognitive radio network, when any two node devices that the same channel is allocated exist within a distance that mutual propagation (communication) is allowed, a communication link using the channel is established between the node devices.
Therefore, in accordance with change of the position of each of the node devices and a channel allocated to each of the node devices, a network topology changes. Moreover, characteristics such as a distance that propagation is allowed, a capacity (e.g., a communication capability, an available bandwidth), and a loss rate (e.g., a packet loss rate) vary for each channel.
Therefore, in a cognitive radio network, by changing a channel allocated to each of the node devices, it is possible to relatively freely change the characteristics (e.g., a communication bandwidth that can be offered, a communication quality, or the like) of a communication network.
For example, between node devices where much communication demand exists, it is possible to configure a communication network so that a communication link having a large capacity is established. In this case, as a channel used by the communication link, it is possible to use a channel having a capacity as large as possible. Moreover, it is also possible to configure a communication network so that relatively many auxiliary communication paths (e.g., a disjoint path) in case of occurrence of a fault can be secured.
Thus, a cognitive radio network needs a technique for determining how to allocate a channel to each of the node devices. As this kind of technique, techniques described below are known.
For example, Non-Patent Document 1 discloses a technique of configuring a group (a coordination group) for regulating channels between a plurality of node devices to which channels similar to each other are allocated and which are located near. In this technique, a channel used by each of the node devices is determined by voting from among available channels so that the number of users sharing the same channel becomes the most.
Moreover, in this technique, each of the node devices has a plurality of radio interfaces. Therefore, in this technique, by allocating channels different for each of the radio interfaces, it is possible to establish many communication links between the node devices.
Further, Non-Patent Document 2 discloses a technique of, when allocating channels to node devices each having a plurality of radio interfaces, assuming communication links existing within a preset distance and using the same channel interfere with each other and allocating the channels so as to minimize the interference.
[Non-Patent Document 1]
    J. Zhao, H. Zheng, G. Yang, “Distributed Coordination in Dynamic Spectrum Allocation Networks,” Proceedings of IEEE DySPAN (New Frontiers in Dynamic Spectrum Access Networks), IEEE, November 2005, pp. 259-268[Non-Patent Document 2]    M. K. Marina, S. Das, “A Topology Control Approach for Utilizing Multiple Channels in Multi-Radio Wireless Mesh Networks,” Proceedings of 2nd International Conference on Broadband Networks, October 2005, pp. 381-390
As mentioned above, in a cognitive radio network, when any two node devices with the same channel allocated are located within a distance which allows mutual propagation (communication), a communication link using the channel is established between the node devices. However, all of the established communication links are not used as a communication path actually.
FIGS. 1A and 1B show two examples of a network topology in a cognitive radio network. FIG. 1A shows a network topology 101 configured by five node devices A to E. To the node devices A to E, channels 0,1, channels 0,1, channels 1,2, channels 0,2 and channels 0,2 are allocated, respectively.
In this example, a communication link is established between node devices to which the same channel is allocated and which are located within a mutual propagation distance. To be specific, communication links are established between the node devices A-B, between the node devices B-C, between the node devices C-D, between the node devices D-E and between the node devices B-D, respectively.
Hereinafter, a communication link established between node devices X-Y will be described as a communication link X-Y. However, in FIGS. 1A and 1B, even when a plurality of communication links are established between certain node devices, a communication link is shown in the same manner as when only one communication link is established.
Further, a link cost is set with respect to each communication link. A link cost is a value representing the cost of transferring data from a node device to another node device adjacent thereto (an adjacent node device). In this example shown in FIG. 1A, link costs set with respect to communication links A-B, B-C, C-D, D-E and B-D are 2, 2, 1, 1 and 4 , respectively.
In path control, generally, a communication path is determined so that the sum of link costs set with respect to communication links configuring a communication path becomes the minimum. Therefore, in the abovementioned example shown in FIG. 1A, as a communication path between the node devices B and D, not a communication path passing through the communication link B-D but a communication path passing through the communication links B-C and C-D is selected because the sum of the link costs is smaller. That is to say, the communication link B-D is a communication link through which traffic does not pass (i.e., data is not transmitted).
Further, FIG. 1B shows a network topology 102 configured by four node devices A to D. To the node devices A to D, channels 1 and 2, channels 0 and 1, channels 0 and 2, and channels 1 and 2 are allocated, respectively. In this example, a communication link is established between node devices to which the same channel is allocated and which are located within a mutual propagation distance.
To be specific, communication links are established between the node devices A-B, etween the node devices B-C, between the node devices C-D, between the node devices D-A and between the node devices B-D, respectively. Link costs set with respect to the communication links A-B, B-C, C-D, and B-D are 4, 6, 1, 1 and 2, respectively.
Therefore, in this example, for the same reason as in the example shown in FIG. 1A, the communication links A-B and B-C are communication links through which traffic does not pass.
Thus, in a cognitive radio network, even when a channel is allocated to a radio interface, there is a case that a communication link using the channel is not set as a use communication link. Herein, a use communication link is a communication link configuring a communication path that a node device uses for transferring data. Moreover, even when a channel is allocated to a radio interface, there is a case that a communication link using the channel is not established.
In a case that a channel is thus allocated to a radio interface wastefully, the following problems arise.
For example, in a case that a channel of a certain frequency is divided in accordance with TDMA (Time Division Multiple Access) and allocated to a plurality of nodes, there is a need to allocate different timeslots to the node devices with the channel allocated, respectively.
In this case, when some of the node devices are not using the channel for communication links set as use communication links, the timeslots allocated to the channel are wasted (are not used for transfer of data). In other words, a timeslot allocated to a channel used for a communication link set as a use communication link is made to be short wastefully.
Further, in a case that there is a channel which is not used for a communication link set as a use communication link, electric power is wasted because electric power is supplied to a radio interface to which the channel is allocated.
Thus, because a node device used in the abovementioned technique is kept in a state that a channel is wastefully allocated to a radio interface, the problems as described above arise.
Such problems also arise in a communication network other than a cognitive radio network. For example, such problems also arise in a multi-hop communication network that performs multi-hop communication.